A control valve is generally used for a continuous control of a liquid or gas flow in various pipelines and processes. In a processing industry, such as pulp and paper, oil refining, petrochemical and chemical industries, different kinds of control valves installed in a plant's pipe system control material flows in the process. A material flow may contain any fluid material, such as fluids, liquors, liquids, gases and steam.
Successful control valve sizing and selection depend on knowing the operating conditions in the system in which the control valve is to be installed. It is known that distinct information on operating conditions very seldom exists. The more assumptions one has to make on flow conditions, the less accurate the control valve sizing is going to be. A common problem is an oversized valve or pump. This means that the valve operates with openings that are too small, within a very narrow opening range and with high installed gain. A high installed gain means that even small changes in the control signal, and respectively in valve travel, effect relatively large changes in flow. To control such a loop accurately is very difficult.
Under operating conditions a control valve is part of a process pipeline. A process pipeline often includes a number of pump-valve systems wherein a pump produces a fluid pressure to provide a fluid flow in the pipeline, and the fluid flow is controlled by a throttling control valve located somewhere in the pipeline after the pump. The pumps are driven by electrical energy. Such throttling flow control means that the “extra” pressure energy produced by the pump is wasted in the throttling control valve. Therefore, it is desired to size the pump-valve system as optimally as possible in order to avoid wasting the pumping energy and to operate the control valve more optimally. A large industrial process may contain hundreds of pump-valve systems, and the amount of wasted pumping energy may be very large. For example, estimation has been presented that the annual total amount of wasted pumping energy in the process industry in Finland may be even 500 MW which would correspond to the energy production capacity of one nuclear power plant. Thus significant savings in energy and cost could be obtained by a correct sizing of the pump-valve systems. Moreover, the wrongly sized pump-valve system results in reduced control performance and control accuracy of the valve.
A process analysis during planning enables a plant design engineer to select a pump based on expected flows in a pipeline. However, actual flows in the process are impossible to predict exactly, and the actual flows typically differ from the estimated designed flows. This may result in a wrongly sized pump. Moreover, when a plant is designed, a pump dimension is often exaggerated in order to secure the operation of the plant. It is also possible to make offline analysis of the pump-valve system. However, such analysis focuses on the identification of pump and system curves and neglects variations of system curve. The analyzed operation period is typically short, while the period should be significantly long (months or years) in order to be representative because the variations in a system curve are sometimes slow, due to fouling, clogging, etc. The high number of pump-valve systems in a plant also makes it laboursome to analyse and recognise inappropriately operating pump-valve systems.
EP0962847 discloses a method and equipment for controlling a pipe network comprising piping, an inverter-controlled pump and at least two control valves. The valve position and the flow through the valve are monitored, and the rotational speed of the pump and the position of each valve are adjusted on the basis to the position and flow data received. The rotational speed of the pump is adjusted to be as low as possible but, at the same time, sufficient for maintaining the gain of the valves. The positions of the valves are adjusted to be as open as possible, the valve opening being, however, for the major part of the time, not more than a certain predetermined portion of the opening of the totally open position. As a result, the energy consumption required for the pumping is decreased and the control accuracy of the valves is increased.
This prior art approach is only applicable to few pump-systems in a plant but fails to provide an universal way to reduce the waste of pumping energy in the plant containing a high number of pump-valve systems.
WO 2011/104419A1 discloses a method for diagnosing a pump-valve system, wherein a valve opening of a control valve and a pressure difference over the control valve during normal operation of the control valve is measured. Then, based the measured valve opening data and the measured pressure data and a stored inherent valve flow coefficient (Cv) characteristic curve of the control valve, an actual pressure difference over the control valve as a function of a flow rate through the control valve, as well as an actual maximum flow rate through the control valve are determined. In another embodiment, a flow rate through a valve is measured by a flow indicator installed in the pipeline, and a based the measured valve opening data and the measured flow rate and a stored inherent valve flow coefficient (Cv) characteristic curve of the control valve, an actual pressure difference over the control valve as a function of the measured flow rate through the control valve, as well as an actual maximum flow rate through the control valve are determined. Finally in both approaches, based on the pressure difference and the actual maximum flow rate, it is possible to determine a potential reduction achievable in the pressure difference, if a pump is resized and the following control valve is adjusted accordingly. The potential reduction of the pressure difference can be reported to a user via a user interface, such as graphical user interface, web-based user interface or voice user interface, or by means of any messaging or data communication mechanism.
A problem related to this arrangement is that pressure sensors are needed at a control valve for measuring the pressure difference, or a flow sensor is needed for measuring the flow rate. Providing new control valves with pressure sensors only for this purpose is costly, and on the other hand, the arrangement is not applicable with existing control valves in a plant. Further, flow sensors are not necessarily available in all parts of the pipeline, and providing flow sensors only for this purpose is costly.